The present invention is related to the field of power distribution between a centrally located power source and a plurality of remote loads. In particular, the invention describes a current limiter circuit for managing the power supplied from a central power supply to a remotely located load such as a telephony load or other remote electronic device. The current limiter monitors current, voltage, internal temperature, and the elapsed time while managing power distribution to meet system requirements for the remote powering of remote electronic devices.
A particularly useful application for the invention of this disclosure is in the powering of a specific type of remote electronic device, an optical network unit ("ONU"). An ONU is a device that is used as an interface between fiber optic telecommunication lines and traditional wires used to provide telecommunication services such as cable television and telephonic services to homes or other buildings.
Typically, several ONUs are positioned in the field to provide telecommunication services to different geographic locations and are provided power from a single local power hub ("LPH"). The ONUs are provided power through a pair of wires from power supplies within the LPH. The LPH could be located up to 6000 feet away from a particular ONU. A separate pair of powering wires is routed to each ONU from the LPH. Commonly, each powering wire pair is powered from an individual power supply within the LPH so that the voltage and current supplied can be closely regulated. Such a system, however, increases the physical size of the LPH and also increases the cost because several individual power supplies are required to power the several ONUs.
One attempt at solving this problem has been to utilize a central power supply and to couple each powering wire pair to the power supply through a dedicated current limiter for that powering wire pair. Such a system is described in U.S. Pat. No. 5,706,157 and is incorporated herein by reference. In the system of U.S. Pat. No. 5,706,157, a single power source feeds a plurality of powering wire pairs that are current limited by an active current limiter for each wire pair.
Current limiters of the type described in U.S. Pat. No. 5,706,157, however, suffer from several disadvantages. They do not provide adequate protection from voltage surges. Because the powering wire pairs to the remote electronic device may be up to 6000 feet long, they are susceptible to hazards such as high-voltage lightning strikes and power cross resulting from power lines falling across the powering wire pair.
Present current limiters are also disadvantaged because they do not regulate voltage drop but rather have a variable voltage drop. Industry standards in the telecommunication field limit the maximum deliverable voltage to power lines to 140 V as a safety precaution to make the power lines safer for workers in the field who may work on the power lines. To ensure that present current limiters deliver no more than 140 V to the load, the voltage supplied by power sources to present current limiters is limited to 140 V plus the minimum voltage drop from the current limiter. Because the voltage drop in the current limiter could be greater than the minimum, the voltage delivered by present current limiters to the power lines could be less than 140 V. For example, if the voltage drop in present current limiters varied from 1.5 to 3 V, the maximum voltage delivered by the power source to the current limiter would be no higher than 141.5 V and the voltage delivered from the current limiter to the power lines would vary between 140 V to 138.5 V. With a regulated voltage drop, power sources could be adjusted to deliver a higher voltage to the current limiter and the current limiter could ensure that the maximum allowable voltage is always delivered to the load. It is important that the highest possible voltage is delivered to the load so that the power delivered the load is maximized: the power available to the load is directly proportional to the square of the delivered voltage. Current limiters of the type described in U.S. Pat. No. 5,706,157 have an unregulated voltage drop.
Additionally, present current limiters are disadvantaged because they do not allow for a periodic release of voltage to line. In applications such as the ONU application, a protector module may be placed between the limiter and the powering pair. The protector module limits surges originating from lightning or other undesirable sources. The protector module also exists at the load. The protector at the load may activate or fire due to a surge and remain on thus shunting power away from the telephony equipment. It would be desirable to have a mechanism for disconnecting the power to allow the fired protectors to reset thus eliminating the need to manually reset the protector.
Further, present current limiters are disadvantaged because they lack thermal protection circuits. The temperature inside of equipment cabinets that house the current limiters could become elevated due to a number of conditions, including excess self-generated heat by the current limiters and equipment failures such as blown fans fuses and heat exchanger failures. If the equipment cabinets' temperature rises too high, the present current limiters' electronics could fail because they lack thermal protection circuits.
Therefore, there remains a need in this art for an improved current limiter. There remains a more particular need for an improved current limiter that provides protection against power surges such as those caused by lightning strikes or power cross. There also remains a more particular need for an improved current limiter that regulates voltage drop. There remains a further need for an improved current limiter that periodically releases voltage. Finally, there remains a need for an improved current limiter that includes thermal protection circuitry.